linux-kernel - Re: [PATCH 4.4 014/134] perf tools: Make perf_event__synthesize_mmap

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Message-ID: <1522340036.2654.20.camel@codethink.co.uk>
Date:   Thu, 29 Mar 2018 17:13:56 +0100
From:   Ben Hutchings <ben.hutchings@...ethink.co.uk>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Cc:     stable@...r.kernel.org, Stephane Eranian <eranian@...gle.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Sasha Levin <alexander.levin@...rosoft.com>
Subject: Re: [PATCH 4.4 014/134] perf tools: Make
 perf_event__synthesize_mmap_events() scale

On Mon, 2018-03-19 at 19:04 +0100, Greg Kroah-Hartman wrote:
> 4.4-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Stephane Eranian <eranian@...gle.com>
> 
> 
> [ Upstream commit 88b897a30c525c2eee6e7f16e1e8d0f18830845e ]
> 
> This patch significantly improves the execution time of
> perf_event__synthesize_mmap_events() when running perf record on systems
> where processes have lots of threads.
> 
> It just happens that cat /proc/pid/maps support uses a O(N^2) algorithm to
> generate each map line in the maps file.  If you have 1000 threads, then you
> have necessarily 1000 stacks.  For each vma, you need to check if it
> corresponds to a thread's stack.  With a large number of threads, this can take
> a very long time. I have seen latencies >> 10mn.
> 
> As of today, perf does not use the fact that a mapping is a stack, therefore we
> can work around the issue by using /proc/pid/tasks/pid/maps.  This entry does
> not try to map a vma to stack and is thus much faster with no loss of
> functonality.
> 
> The proc-map-timeout logic is kept in case users still want some upper limit.
> 
> In V2, we fix the file path from /proc/pid/tasks/pid/maps to actual
> /proc/pid/task/pid/maps, tasks -> task.  Thanks Arnaldo for catching this.
> 
> Committer note:
> 
> This problem seems to have been elliminated in the kernel since commit :
> b18cb64ead40 ("fs/proc: Stop trying to report thread stacks").
[...]

I don't think so.  It looks like this was fixed by commit 65376df58217
("proc: revert /proc/<pid>/maps [stack:TID] annotation") which we
already have in 4.4-stable.  But older branches (3.16, 3.18, 4.1) don't
have that and probably should do.

It looks like commit b18cb64ead40 ("fs/proc: Stop trying to report
thread stacks") is also a candidate for stable.

Ben.

-- 
Ben Hutchings
Software Developer, Codethink Ltd.